This invention relates to eddy current instruments for the non-destructive detection of flaws in electrically conductive articles and more particularly to the detection of flaws in the regions of electrically conductive material surrounding fastener holes.
Eddy current instruments of the type utilizing a source or exciting coil for inducing an alternating magnetic field in a material and a pick-up coil for detecting the magnetic field induced are widely used for locating flaws or imperfections in metallic materials. An eddy current instrument of this type detects a flaw by detecting the metal loss due to the flaw. The localized presence of a flaw or crack in the metal upsets the otherwise normal eddy current distribution. This abnormal apportionment of eddy current creates a resultant change in magnetic flux which is sensed by a change in the inductance of the pick-up coil.
Structural wing skins on cargo aircraft are spliced to each other in an overlapping configuration such that the skins are held together by means of fasteners mounted through holes in both skins. Fatigue damage can occur at the fastener hole in either layer of skin. Presently, two choices exist for the inspection of fastener holes in such structure. Either the fastener can be removed and the hole inspected with conventional eddy current techniques or shear wave ultrasonic methods can be utilized with the fastener intact. Neither choice is entirely satisfactory.
Conventional eddy current equipment is available with cup core type probes designed for the detection of large cracks emanating from fastener holes in second or interior layers. These existing probes interrogate the entire fastener hole during inspection and consequently are not very sensitive to the presence of small cracks. Large cracks can be detected without removing the fastener but detection of small cracks require removal of the fastener. If the structure to be tested has a large quantity of such fasteners, such as an aircraft's wings, the removal and re-installation of fasteners is costly and a potential source of structural damage. Ultrasonic shear wave inspection is an effective method of inspecting the faying surface region of the outer skin (layer) but it is considerably less effective in the region of a fastener hole countersink. When a faying surface sealant is present, the ultrasonic shear wave technique has the potential of penetrating into the inner layer, but the reduction to practice of this inner layer inspection has not occured. When this technique is developed, the faying surface side of the inner layer would not be as effectively inspected as the opposite side, and many structures exist that do not use sealants in a multi-layer configuration. Obviously, then, the requirement exists for an inspection technique that would complement the ultrasonic shear wave technique as well as not be dependent on the presence of sealant for inner layer inspections.
U.S. Pat. No. 3,166,710 to Schmidt discloses an apparatus including a segmented pick-up coil which is formed from a plurality of individual arcuate shaped coil segments disposed to form a single circular coil. This device is useful for detecting flaws in tubular structures such as a pipe, however, its applied magnetic field distribution renders the invention unsuitable for inspecting the wings of an aircraft.